In existing single-vessel kraft digesters, especially hydraulic digesters, there are sometimes problems which concerns difficulties in obtaining a uniform cooking. In order to achieve a uniform and selective cooking, it is desirable to be able to impregnate the chips in the upper part of the digester with hot liquor, possibly including black liquor, which has a relatively high content of sulfide ions and a low content of hydroxide ions. However, a high temperature at the top of the digester leads to a high temperature in the transfer circulation which conveys the chips from a so-called high-pressure feeder, that is to say a pressure sluice which transfers the chips together with liquid from a low-pressure system to a high-pressure system. Thereafter, the liquid is in the main separated from the chips with the aid of a top separator at the top of the digester and is returned to the high-pressure feeder. If this returned liquid has a temperature which exceeds the boiling point in the low-pressure system, the liquid will flash in the high-pressure feeder, said flashing manifesting itself in the form of bangs.
From WO 94/23120 (Collins) it is known to thermally insulate the high-pressure feeder from the digester by means of replacing a first, relatively cold liquid with a second, relatively hot liquid outside the digester, in the transfer line from the high-pressure feeder to the digester. This change of liquids is preferably carried out with the aid of a free-standing, upward-feeding top separator. The first, relatively cold liquid is returned to the high-pressure feeder, as a result of which a first circulation is obtained within the transfer circulation. The second, relatively hot liquid consists to a large extent of liquid from the top of the digester, which liquid is further heated up before being supplied to the liquid exchanger. By means of the return of liquid from the top of the digester to the liquid exchanger, a second circulation is obtained within the transfer circulation.
From WO 96/34143 (Oulie et al.) is known a method similar to that of WO 94/23120 but including black liquor impregnation by addition of hot black liquor to the second circulation.
Both of the methods described in these two WO-applications are well functioning methods. They have, however the draw-back of requiring the installation of a relatively expensive liquid exchanger which might include the risk of clogging.
The object of the present invention is to provide an improved method for continuous cooking of fiber-containing cellulose material, preferably softwood, which method eliminates the abovementioned disadvantages. The invention is especially suitable for use in conjunction with the conversion of existing single-vessel digesters to include high temperatures in the upper part of the digester, for example hot black liquor impregnation in the concurrent zone. By use of the invention, the temperature in the upper part of the digester is easily controlled.
This is achieved by the method according to patent claim 1, which means that a constituent flow of liquor is led from the return line of the transfer circulation, is heated in one or more beating arrangements and is fed via one or more inlets into the digester at a location downstream of the outlet for the return line of the transfer circulation.
A further aspect of the invention is that there is a temperature difference between the transfer line at the inlet into the digester and the zone immediately downstream of said one or more inlets which temperature difference is more than 5.degree. C., preferably more than 10.degree. C., and still more preferably more than 15.degree. C.
According to a further aspect of the invention, one or more additional liquids are added to said constituent flow before its addition to the digester. These liquids may constitute of black liquor which can be added to said constituent flow in a quantity in excess of 40%, preferably 50%, and more preferably 60%, of the total quantity of liquid, by means of a part of the black liquor which is extracted from the digester being recirculated to said constituent flow. Yet another possibility is that white liquor is added to said constituent flow. Naturally, both black liquor and white liquor or any other chemical which is beneficial for the cooking reslut can be added at the same time.
According to a further aspect of the invention, the first screen section which is arranged in the digester, downstream of said one or more inlets for the constituent flow, may be the extraction screen section for extraction of black liquor.
According to a further aspect of the invention, said one or more inlets for the constituent flow are situated less than 5 metres, preferably less than 3 metres and more preferably less than 1 metre downstream of said outlet for the return line.
One advantage of the invention is that a relatively high temperature can be achieved in the upper part of the digester, without the risk of steam production, flashing, in the high pressure feeder.
Another advantage is that the invention results in a greatly improved heat ecomony.
Another advantage is that the number of circulations, including screens can be reduced, since heating largely takes place externally, outside the digester.
When converting older, existing digesters, the present invention affords the possibility of achieving a uniform and selective cooling, with accompanying improved pulp quality. This is especially true in connection with the introduction of an extra circulation with white liquor charging and high flow in the countercurrent washing, by which means so-called isothermal cooking ITC.TM. in accordance with patent application SE 9203462 can be achieved.